P0420-Catalyst below threshold. I had the same code. I spent $2K. No doubt, you have excessive blowby from worn piston rings and valves. That Prism is durable. You shouldn't have to spend $2K again for several years. Look into "low ash" motor oils that run clean. Sooner or later you will have to invest in a new car. They are getting better and better, meaning cleaner running, more fuel efficient, and with greater durability. I offer this conjecture as someone who has logged over 2 million miles in the last 35 years, in passenger vehicles, and while dealing with California smog laws. I am glad I don't have your rust issues. I hope this helps.